56 EDMUND B. WILSON 



the 34 cases may be taken as characteristic of the dividing sper- 

 matogonia, and that it occurs with a high degree of constancy. 



Six of these groups, from the best that could be found, three 

 from each testis, are shown in fig. i, a-/. These have been se- 

 lected particularly to show the different positions of the three 

 small chromosomes. The latter appear to follow no rule what- 

 ever, the three lying anywhere in the metaphase-plate; and all 

 may be separate, all together, or two together and one separate. 

 This is an interesting and significant fact, because in the first 

 spermatocyte-division, as described beyond, the three are always 

 associated to form a triad element which invariably occupies the 

 same position in the chromosome-group (see p. 58). 



For the sake of comparison, four sperm atogonial groups from 

 other individuals are here reproduced (from my fifth Study). 

 Two of these (fig. i, i, j] are from femoratus, No. 29, which has 

 the typical diploid group of 22 chromosomes, including but two 

 small (m-chromosomes.) The other two (fig. i, g, h) are from 

 terminalis, No. 2, which has 23 chromosomes, including two m- 

 chromosomes and one small supernumerary 3 . As will appear 

 beyond, this latter chromosome is wholly different in nature from 

 the third small chromosome in individual No. 64, though indis- 

 tinguishable from it by the eye in the spermatogonial groups. 



As the figures show, the larger chromosomes in No. 64 show 

 well marked size-differences, and in most of the groups a largest 

 and second largest pair are usually fairly evident; but it is impos- 

 sible to pair all of the chromosomes accurately by the eye. It 

 is, however, obvious that not more than 18 of the 19 can be equally 

 paired. One of them must either have no proper mate, or it must 

 form a very unequal pair with the third small chromosome. The 

 following possibilities must, accordingly, be considered: 



1. The nineteenth large chromosome and the third small one 

 are respectively a large and an abnormally small idiochromosome 

 which form a pair of synaptic mates, or 



2. The nineteenth large chromosome is an odd or accessory 

 chromosome, without a synaptic mate, while the third small one 

 is similar to a small "supernumerary" or 



* Cf: Photo. 29, Study V. 



